only iterate through cols if cols is not None.  Fixes #417. Tests pass again on my local machine.
only iterate through cols if cols is not None.  Fixes #417. Tests pass
again on my local machine.

pauldeng/nilmtk,AlexRobson/nilmtk,nilmtk/nilmtk,nilmtk/nilmtk,HarllanAndrye/nilmtk,mmottahedi/nilmtk,josemao/nilmtk,jaduimstra/nilmtk